engine: block editing migraion fields while VM !Down

While the VM isn't down block editing migration and
pinToHost fields in engine, and add a tooltip in UI to
explain that.

Change-Id: If9fb2fc0755317a8d4be7628b56966d82e9adc3a
Bug-Url: https://bugzilla.redhat.com/735260
Signed-off-by: Gilad Chaplik <gchaplik@redhat.com>